The present invention relates in general to methods and apparatus for monitoring cardiovascular regulation and in particular to methods and apparatus for heart rate spectral analysis.
Changes in cardiovascular regulation associated with congestive heart failure include attenuation of activity in the parasympathetic division of the autonomic nervous system, enhancement of activity in the sympathetic division of the autonomic nervous system, cardiac catecholamine depletion, down regulation of the beta-receptor system, increased renin-angiotensin system activity, and alteration of baroreceptor function. All of these regulatory changes require either specific clinical manipulations, such as a stress test, a Valsalva maneuver, or the like, and/or invasive maneuvers, such as cardiac biopsy, plasma catecholamine measurement, or the like, in order to determine the extent of regulatory dysfunction and its impact upon the clinical state of the patient and upon prognoses for the patient. These procedures are time consuming, and generally do not permit the formation of a clinical judgment and subsequent action within the timeframe of the course of treatment for critically ill patients in an Intensive Care Unit.
Fluctuations from heartbeat to heartbeat in measured properties of the circulatory system reflect both the presence of a variety of naturally occurring physiological disturbances of the circulatory system homeostasis, and the dynamic response of cardiovascular control systems to these disturbances. For example, the cyclic variation in intrathoracic pressure which accompanies breathing mechanically affects the return of venous blood to the heart and also affects blood pressure in pulmonary vessels and in the aorta. The variation in intrathoracic pressure is also coupled to a cyclic variation in heart rate through a neural mechanism mediated by the central nervous system. Furthermore, the resulting cyclic variation in arterial blood pressure impinges on heart rate through a reflex, known as the baroreceptor reflex, which is mediated by the autonomic nervous system. Disturbances in cardiovascular homeostasis also occur with fluctuations in the resistance of peripheral blood vessels as vascular beds regulate local blood flow to match supply with demand. These fluctuations in peripheral resistance may perturb central blood pressure and through the baroreceptor reflex, may also lead to a compensatory variation in heart rate.
Many types of medical instruments exist for studying heart rate variability. The instantaneous rate-meter is perhaps the earliest such instrument. This meter measures each RR interval through analog or digital circuitry and displays the instantaneous heart rate.
An improvement in the rate-meter is achieved by performing first order statistical evaluation on the RR-intervals. With mini- and micro-computer systems histogram displays of RR-interval differences may be generated along with their mean and standard deviations.
Another technique for heart rate variability analysis involves the study of spectral content of the instantaneous heart rate time series. In one approach to spectral analysis in animals, the computations are done on a computer. Akselrod, et al., Science, 213, 220-222 (1981) Hyndman, et al., Automedica, 1, 239-252 (1975). Such systems analyze data recorded on magnetic or punched tape. However, not only do these systems introduce additional errors during the recording process, they do not perform in real time. Furthermore, these systems are not multichannel in nature.
A Sparse Discrete Fourier Transform algorithm which may be implemented on a personal computer (CBM 2016) and which may perform on-line monitoring of heart rate variability, based on a low pass filtered cardiac event series is disclosed in Rompelman, et al., IEEE Trans. Biomed. Engineering, BME-29, 503-510 (1982). A specialized hardware device also exists for low pass filtering the cardiac event series by a stepwise convolution to create the low pass filtered cardiac event series. Coenen, et al., Medical and Biological Engineering and Computing, 15, 423-430 (1977). Nevertheless, these instruments possess a limited band width and a limited frequency resolution capability.
There exists a need for an instrument which provides multi-channel spectral analysis of an instantaneous heart rate and of a respiratory activity time series. There also exists a need for an instrument wherein such calculations are performed in real time at the bedside.